1. Field of the Invention
The present invention relates to a complementary insulated-gate field effect transistor integrated circuit and a method of manufacturing the integrated circuit.
2. Description of the Prior Art
In manufacturing complementary insulated-gate field effect transistor integrated circuits (CMOSIC's), the device characteristics vary due to changes, such as those in temperature, manufacturing precision, and operations, in the process steps as the degree of device integration is increased, which leads to a disadvantage that the yield of products is lowered.
The changes in device characteristics include, for example, a change of a threshold voltage, Vt of a transistor.
In general, the threshold voltage Vt of an MOS transistor is expressed as follows: EQU Vt=.phi..sub.MS +2.phi..sub.F -(Q.sub.SS +Q.sub.B +Q.sub.imp)T.sub.ox /.epsilon..epsilon.'
where,
.phi..sub.MS =Work function of gate electrode material PA1 .phi..sub.F =Fermi potential PA1 Q.sub.SS =Surface state density (1/cm.sup.2) PA1 Q.sub.B =Bulk electric charge (1/cm.sup.2) PA1 Q.sub.imp =Impurity concentration of ions implanted in semiconductor substrate (1/cm.sup.2) PA1 T.sub.ox =Insulation film thickness PA1 .epsilon.=Dielectric constant of vacuum PA1 .epsilon.'=Relative dielectric constant of insulation film material
Consequently, if the changes of the Vt value due to the short-channel effect, narrow-channel effect, and the substrate bias effect are ignored, the variations of the Q.sub.imp and T.sub.ox are the main factors among the terms of the expression above which are related to the change of Vt.
The process step causing the change of the threshold voltage Vt is different between the p-type MOS and n-type MOS transistors in an IC having a very fine structure. In an n-type MOS transistor, the change of the thickness of the insulated film for the gate, T.sub.ox and the change of the amount of implanted ions, Q.sub.imp controlling the threshold voltage Vt on the portion of silicon substrate used as a channel below the insulated gate layer are the predominant factors to change the threshold voltage Vt; whereas, in a p-type MOS transistor, the change of the length of gate electrode, L and the change of the amount of implanted ions, Q.sub.imp in the portion of substrate to be used as a channel below the insulated gate layer are the main factors to change the threshold voltage Vt, Y.AOKI et. al., IEEE TRANS on Electron Device, vol. ED-31, 1984, p. 1462. Particularly, in an n-type MOS transistor, the thickness of the oxidized layer for the gate, T.sub.ox primarily changes the threshold voltage Vt; whereas, in a p-type MOS transistor, the gate length L mainly causes the threshold voltage Vt to change.
Consequently, in the process of manufacturing CMOS IC's having a fine structure, the change of the threshold voltage Vt due to the variation in the thickness of the gate layer T.sub.ox of an n-type MOS transistor and that in the gate length L of a p-type MOS transistor are required to be minimized.
Conventionally, the-ion implantation condition for the p-type and n-type MOS transistors is set such that the ion acceleration energy is determined so that the ion implantation range R.sub.p simply exceeds the thickness of the insulated gate layer T.sub.ox as represent by the following expression. EQU R.sub.p &gt;T.sub.ox
Consequently, in an n-type MOS transistor, when the energy for accelerating the ions to be implanted increases, the amount of the ions implanted into the semiconductor substrate Q.sub.imp does not change so much even if the thickness of the insulated gate layer T.sub.ox is changed, which leads to a disadvantage that the threshold voltage Vt varies primarily depending on the change in thickness of the insulated gate layer T.sub.ox.
On the other hand, in a p-type MOS transistor, the ratio of the change of threshold voltage to the change of the thickness of the gate formed with an insulating material is relatively small when compared with the n-type MOS transistor; it is therefore necessary to mainly reduce the change of the amount of the ions implanted in the semiconductor substrate Q.sub.imp.